1. Field of the Invention
This invention relates to a process for forming leakage resistant seals between metal parts of electrochemical cells exposed to strong alkaline solutions. More particularly, it relates to a method for sealing the interface between the metal container and the metal cover of electrochemical cells containing alkaline electrolyte.
2. Description of the Prior Art
It has been found very difficult to form an electrically insulated liquid type seal between the metallic container and the metal cover of electrically chemical cells of the types using an alkaline electrolyte. Typical cell systems where this problem is encountered include mercury-cadmium cells, nickel-cadmium cells, nickel-zinc cells, silver oxide-zinc cells, silver oxide-cadmium cells, mercuric oxide-zinc cells and alkaline manganese dioxide-zinc cells. Although the problem is essentially in sealed rechargeable cells, it is also of major importance in the design of primary type batteries. This is particularly true of high quality batteries where the supplier endeavors to provide a high capacity battery capable of delivering its full output after extended storage.
In a typical cell, the active cell parts are assembled in a seamless metal cup. A metal cover forms the cell closure and the cup and cover form the two terminals of the cell. It is necessary to provide an insulating seal between the two metal parts and for this purpose a ring or grommet of non-conductive material has been used. This grommet may be shaped to slip over or be injected molded over the edge of the metal cover and is clamped to the metal cup by flanging the edge of the cup over the grommet. The material from which the grommet is made must be inert to the electrolyte contained in the cell and to the cell environment. It must be resilient and flexible and it must be resistant to cold flow under the pressure of the seal even under long periods of time. Materials such as nylon and polypropylene have been found to be suitable materials for this insulating gasket.
Unfortunately, it transpires that electrolyte, particularly alkaline electrolyte, has a strong tendency to creep on the metallic surfaces. This tendency is accentuated when an electrical potential is present. In actual experience it will be found that creepage is accentuated on a metal part which is in contact with and electrically negative to an alkaline electrolyte. The creepage is harmful in a number of ways. It represents a loss of electrolyte from a cell which at best has a very limited electrolyte supply. The electrolyte that leaks out can be harmful as it is a strong chemical. Also, in the case of alkaline electrolytes there is a reaction with air to form a white deposit. This deposit detracts from the appearance of the cell and can produce customer resistance.
Numerous designs of seals have been proposed for overcoming the leakage problem. These range from improved mechanical clamping of the grommet to seals based upon ceramic or metal fusion. One solution has been to coat the metal parts or grommet with an asphaltic compound. Unfortunately, asphaltic compounds slowly flow under the pressure exerted by the gases contained in the cell which results in eventual leakage. Also, the sticky asphaltic surface is difficult to handle during manufacturing processes. In particular, it tends to pick up dirt and lint from the air and these impurities can interfere with proper sealing.
Common adhesives have been unsatisfactory. For example, epoxy resins applied to metal, while making a perfect and permanent seal with nonpolar liquids, are found with alkaline electrolytes to be lifted off of the metallic surface by the greater surface forces of the electrolytes. Also, usual epoxies tend to be brittle. The stresses imposed on epoxy seals during the operation of closing the cell seem to create cracks or other capillary leakage paths so that the seal made with the normal epoxies have not been as reliable as seals made with asphaltic seal coatings.
U.S. Pat. No. 3,922,178 issued Nov. 25, 1975 to Carol Winger and U.S. Pat. No. 3,712,896 issued Jan. 30, 1973 to Ralph Feldhake disclose the application of a fatty polyamide and a fatty polyamide epoxy mixture to either the grommet or to the metal cover prior to injection molding the nylon grommet to the metal cover. These compounds provide an adequate seal but require extensive equipment and processing for successful application and use. What is required, is a method not requiring extensive equipment and having ease of processing which will provide an insulating seal.